In our earlier Autopen® device, a rotatable dose setting knob attached to the rear end of the pen injector is connected to a hollow drive sleeve that carries an annular ratchet surface at its front end which engages a corresponding ratchet surface on a threaded drive collar. The threaded drive collar is threadedly engaged with the stem of a plunger so that rotary movement of the drive collar is converted into a linear advance of the plunger. A trigger can be moved to release the drive collar for rotary movement. During a dose setting routine, with the drive collar held against rotation by the trigger, a dose is dialled in by rotating the dose setting knob and the drive sleeve against a spring bias provided by a main drive spring. The dose setting movement is unidirectional only due to the ratchet action between the drive sleeve and the drive collar. Upon release of the trigger, the drive collar rotates by an angular amount equivalent to the angular amount initially dialled in, and the plunger is advanced by the corresponding amount to deliver the dose of the required number of units.
This device performs extremely well and enjoys considerable success but we have identified a number of improvement opportunities. In the above device, in setting a dose the user winds up the spring to provide the motive force that is needed for the next dose. This means that it can be awkward for those of limited dexterity to exert sufficient grip to rotate the dose setting knob against the bias of the drive spring. Also in this design, because the action of dose setting winds up the drive spring, a ratchet action is required so that when the user releases their grip on the dose setting knob, the knob stays in that position and does not immediately return to the zero position under the influence of the drive spring. To allow dose setting reversal in case of setting overshoot, a bi-directional ratchet mechanism is then needed, or otherwise a disconnection mechanism is required, either of which complicate the device.